The ICC International Maritime Bureau warned today that attacks by pirates against tankers in Indonesian waters could lead to an environmental disaster.

Vulnerable vessels, such as a gas tanker and chemical tankers, have been targeted by heavily armed pirate gangs. It has been widely reported that many of the attacks have been by Aceh rebels fighting for independence from Indonesia.

In its latest quarterly piracy report, the IMB reported that numbers of piracy attacks on shipping throughout the world reached a record 344 in the first nine months of 2003, with Indonesian waters remaining the most dangerous.

The IMB and its Kuala Lumpur-based Piracy Reporting Center said the spate of attacks on small tankers by gangs of heavily armed pirates aboard fishing boats and fast craft had heightened tensions in the area.

"In most cases the attacks are thought to be led by Aceh rebels," the IMB said. Their main purpose was to raise money to fund their separatist fight by holding hostages for ransom, it added.

Investigators were concerned that most of the attacks were within the same 30 nautical mile radius of the rebel stronghold in Sumatra.

In July, there were three attempted boardings in quick succession off the Sumatra coast in the Malacca Straits, one of the world's busiest sea lanes. Pirates raked an LPG tanker, a gas tanker and an oil tanker with automatic fire, but in each case the crews managed to thwart the attacks.

More recently, a fully laden oil tanker, the Penrider, was attacked by pirates wearing military-style fatigues and carrying assault rifles. The IMB said the attack bore all the hallmarks of the Aceh rebels.

IMB Director Captain Pottengal Mukundan said: "If Aceh rebels are behind the Penrider attack, we need to know...Politically motivated pirates are prepared to take greater risks to further their cause. We have seen the devastation that results from this in other parts of the world."

The IMB report said: "A spate of attacks against small tankers in the Malacca Straits by gangs of heavily armed pirates aboard fishing boats and fast craft has heightened tension in the area. These have led to repeated warnings from the IMB and its Piracy Reporting Centre, which fear an environmental disaster in these restricted waters if a larger tanker is subjected to the same type of attack."